Char dominates black carbon aerosol emission and its historic reduction in China

Emission factors and inventories of black carbon (BC) aerosols are crucial for estimating their adverse atmospheric effect. However, it is imperative to separate BC emissions into char and soot subgroups due to their significantly different physicochemical properties and potential effects. Here, we present a substantial dataset of char and soot emission factors derived from field and laboratory measurements. Based on the latest results of the char-to-soot ratio, we further reconstructed the emission inventories of char and soot for the years 1960–2017 in China. Our findings indicate that char dominates annual BC emissions and its huge historical reduction, which can be attributable to the rapid changes in energy structure, combustion technology and emission standards in recent decades. Our results suggest that further BC emission reductions in both China and the world should focus on char, which mainly derives from lower-temperature combustion and is easier to decrease compared to soot.


Text S1. Sampling details
We conducted stove-top field measurements of RS in 122 rural households in 10 eastern provinces (Shanghai, Jiangsu, Zhejiang, Anhui, Hubei, Shandong, Hebei, Jilin, Heilongjiang, Neimeggu) from September 2018 to July 2022.To reflect the real combustion situation in rural areas, representative fuels and stoves were used in the combustion measurements based on the daily cooking and heating habits of residents.
Solid fuels were divided into biomass (straw and firewood) and coal.Straw is divided into nine categories depending on their type (corn, corn cobs, soybeans, rice, cotton, sorghum, reeds, peanuts, and bamboo), firewood is divided into branches and trunks depending on the part selected for burning, while coal is divided into chunk coal and honeycomb coal depending on its treatment process.Biomass is burned uniformly in brick stoves, while chunk coal and honeycomb coal are burned in separate stoves (traditional iron stove (TIS), improved iron stove (IIS) and honeycomb briquette stove (HBS)).Brick stoves is about 50 cm high and equipped with 1-3 iron pots.TIS and IIS are mainly used for coal heating.IIS has a larger internal space than TIS.In addition, the side windows of the IIS can be opened for fuel and air supply to achieve higher combustion efficiency.HBS use only honeycomb briquettes as heating fuel.They are about 40-60 cm high and have a cylinder chamber in the middle which can burn 3-4 honeycomb briquettes.The exact shape and photos of the stoves can be found in the previous articles of our group (1).Flue gases emitted from the stack are sampled and measured using a mobile dilution sampling system consisting of a dilution channel, two flue gas analyzers (GA21plus, Madur, Austria) and sampling instruments (Fig. S2).A portion of the flue gas emitted from the stack is drawn into the dilution channel and mixed with clean air filtered with polypropylene fibers of 1 μm pore size for cooling and dilution.The pre and post dilution CO and CO2 concentrations are measured online by two flue gas analyzers to obtain the actual dilution ratio, which is 10-20 in the sampling.
Two different references were consulted for the classification and sampling of onroad vehicles and construction machinery respectively (2,3).The actual emissions of gasoline and diesel vehicles with different emission standards (China-III, China-IV, China-V and China-VI) under different driving conditions (idling speed; suburban main road driving; high speed road driving) were collected between December 2020 and March 2022 using follow-up sampling (detailed in Tables S5 and S6).At the same time, particulate matter was collected from the exhaust of 6 forklift trucks of different tonnages and aftertreatment technology levels (with or without diesel particulate filter (DPF)) under different driving conditions (idling, unloaded and loaded) during January-December 2021 using the same sampling method, detailed in Table S7.Specifically, we combined a vehicle emission sampling system to collect particulate matter emissions.
The equipment is mounted on a separate sampling vehicle for each sampling session, and the sampling vehicle follows the subject vehicle at all times.The equipment is powered by a generator on the sampling vehicle to avoid overloading the engine.A homemade dilution sampling system was used to control the dilution and sampling flow rate between the sampling vehicle and the test workshop (Figure S3).The pre-dilution and post-dilution exhaust gases were monitored at all times by two GA21 exhaust gas analyzers and flow meters to determine the instantaneous dilution ratio.A vacuum pump with active flow control is used at the end of the post-dilution sampling channel to ensure a flow rate of 40 L/min through the quartz membrane.A total of 76 particle samples were collected from three ocean-going vessels (OGV, an Aframax oil tanker, a Newcastlemax bulk carrier and a Capesize bulk carrier, detailed parameters in Table S8) between December 2020 and August 2021(4).All samples were collected under steady-state navigation of the OGV, and the corresponding sailing parameters including: engine load (25%, 50%, 75%, 100%), engine power rating (different power of main engine and generator) and fuel type (Marine gas oil and Heavy fuel oil).Specific sampling and dilution system is built with reference to (5,6).
The real-world measurement of ICB and PCB were conducted in Hebi City, Henan Province, from 1 to 5 November 2018.Hebi City is one of the major air pollution transmission channels in the Beijing-Tianjin-Hebei region ("2+26" cities).In summary, for ICB, we collected emission samples from a coal-fired boiler, two coking plants, and two pharmaceutical factories in the vicinity of Hebi City, based on the local industrial source emission inventory provided by the Hebi Environmental Protection Bureau.As for power plant sources, we selected two representative local biomass power plants and a coal-fired power plant for sampling.The samplers were installed at the flue gas outlet after the dust removal.
To further illustrate the formation mechanism and the influence of combustion condition on char and soot formation in RS, laboratory combustion experiments of biomass and coal were conducted through multiple combustion conditions.The quartz tube furnace sampling is done with a self-built quartz tube furnace combustion system, which consists of four parts: air supply module, combustion control module, flue gas dilution module, and sampling module (Fig S3).For the specific structure, please refer to the literature (7).Six solid fuels, including rice straw, corn straw, wheat straw, pine wood, poplar wood and Xuzhou coal (from Xuzhou City, Jiangsu Province, China) were used quartz tube furnace combustion.For each experiment, 1 g of dry fuel powder (60-80 mesh) was burned at different ignition temperatures (defined as furnace temperatures: 300, 400, 500, 600, 700, 800 and 900°C) with different oxygen supply conditions (10.5% vs. 21% O2).Before each combustion experiment, the furnace was heated to the setting temperature.A 1g fuel sample was weighed and placed onto the ceramic crucible.The ceramic crucible was first hung in the low temperature zone, and then was lifted to the combustion zone.The high-purity of compressed air (flow of 5 L/min) was let into the quartz tube from the bottom, and then flue gas rose through the quartz tube and entered the dilution sampling system.The diluted flue gas was subsequently introduced to a filter sampler (quartz fiber filter; d = 90 mm) to collect the total suspended particles.
Each combustion experiment was repeated twice for all fuel/temperature combinations to collect samples.

Text S2. Calculation of emission factors (EFs) and modified combustion efficiency (MCE)
EFs were calculated by dividing the emission by the mass of the fuel consumed, and expressed as grams of emission per kilogram of consumed dry fuel (g•kg −1 )(8).
For particulate pollutants (i.e.OC, BC, Char and Soot), the EFs were calculated as: Where EEp is the EF of particulate pollutants for the specific crop residue;   is the mass of pollutants collected on the filter;  −ℎ is the total volume of exhaust flowing through the chimney during the experiment (m 3 ) at standard temperature and pressure; Q is the sampling volume through the filter (m 3 ) at standard temperature and pressure; Mfuel is the mass of burned fuel (kg, dry basis); and DR is the dilution ratio in the dilution sampler, which was determined using the measured stack, diluted, and background CO2 concentrations (i.e.CO2,Stk, CO2,Dil and CO2,Bkg, respectively), where: For CO2 and CO, the EFs were calculated using online monitored concentrations as follows: where Cx,Dilute is the average concentration (molar fraction) measured in the dilution sampler; Vx is the molar volume of gas at standard temperature and pressure (0.0224 m 3 ) and Mx is the molecular weight of species x (g•mol −1 ).

S10
When CO2 and CO were measured, the MCE was reported as: Where ∆[ 2 ] and ∆[] are the excess molar mixing ratios of CO2 and CO, respectively.

Text S3. Weighted average method
It is necessary to normalize the EFEC and RC/S of the broad categories of sources from the measurements in this study in order to calculate the EFEC and RS/C of the population of individual sources.For biomass, five types of straws that account for more than 90% of total straw burning in China (17.5% for rice, 19.5% for wheat, 39.1% for corn, 5.4% for beans, and 9.3% for oil crops) were combined into straw using their weighted average results overall results (9).The total fuel use ratio of firewood to straw was about 1.2:1, and the overall result of combining straw and firewood into biomass was this weighted result (10).A weighted average result for coal was obtained by considering a total fuel usage ratio of lump coal/briquette coal of 4:1 (11,12).For the three driving states of the motor vehicle, the results of high speed: normal speed: idle speed = 5:4:1 are weighted and averaged, and the mathematical average method is adopted in gasoline and diesel workshops with different emission standards (13).For OGVs, both the generator and the main engine are kept on throughout the journey, and the usage of heavy fuel oil (HFO) is about 4 times that of marine gas oil (MGO).Therefore, the ship emissions are weighted with the result of HFO: MGO = 4:1 (5).

Text
S1 Sampling details Text S2 Calculation of emission factors (EFs) and modified combustion efficiency (MCE) Text S3 Weighted average method

Figure S1 Figure S2
Figure S1 Historical data of emission factors (EF) of black carbon (BC) vs. measured data in this

Figure S3
Figure S3 Emission factors (EF) and char to soot ratio of modified residential fuel

Table S1
Mass-based emission factors (EFs) and char to soot ratio (RC/S) from different BC emission sectors

Table S2
Comparison of emission factors (EFs) of black carbon (BC) between previous studies and our studyTableS3Comparison of emission factors (EFs) of char to soot ratio (RC/S) between our study and previous studies

Table S4
Annual emission of black carbon (BC), char and soot in China (Unit: Gg) from 1960-2017 based on the latest emission inventory

Table S5 Technical
Parameters of the Test diesel vehicles

Table S6 Technical
Parameters of the Test gasoline vehicles

Table S7 Technical
Parameters of the Test forklift vehicles

Table S8 Technical
Parameters of the Test Ocean going vessels

Table S1
Mass-based emission factors (EFs) and char to soot ratio (RC/S) from different emission sectors

Table S2
Comparison of emission factors (EFs) of black carbon (BC) between

Table S4 .
Annual emission of black carbon (BC), char and soot in China (Unit: Gg) from 1960-2017 based on the latest emission inventory

Table S5 .
Technical Parameters of the Test Diesel Vehicles

Table S6 Technical
Parameters of the Test Gasoline Vehicles

Table S8 Technical
Parameters of the Test Ocean Going Vessels